Monday, August 12, 2013

My New Telescope: Ordered and Coming Soon

I presently own several telescopes. The one that sees the most use is my 8" homemade Dobson. It's built pretty close to John Dobson's plans, including a cedar spider for the secondary mirror.

I built that telescope about 11 years ago alongside my oldest daughter building her first telescope. It gets used the most for two reasons: It's the telescope I use for public star parties, and it's just plain easy to haul out, set up, and start looking at stars.

For star parties, it's the perfect scope. It's rugged, so it hauls well. It's easy to set up. It's stable. It stays on target and is easy to put back on target. There are no tripod legs for inexperienced observers to trip over. People like seeing home made telescopes. It works well at low powers, and delivers acceptably good images.

Once, it delivered excellent images. But that day is gone. The mirror has had secondaries dropped on it twice in recent years (no cracks, fortunately) and the coating is showing its age. It's still good enough for star parties, but for my personal use, it's not doing everything I'd like.

I also have a sub-8" Meade Newtonian on a German Equatorial Mount. The optics started out less good than my home made scope. When I made my own telescope mirror for the Dobsonian, I tested the mirror on this one. The outer 1/4" or so of the mirror was seriously out of whack. It caused a lot of reduction in image quality. The rest of the mirror was fine, so I put a flat black cardstock ring over the bad part of that mirror. It sharpened up the image a lot.

The mount, however, is large and difficult to move. It's a project. Just the mount on this scope is heavier and more complex to travel with than the entire Dobson. So falling back on this scope as my primary personal scope wasn't really an option. Too much of my observing happens away from home, at present.

A New StellarVue

I've been lusting after the Stellarvue SV115 telescope for about three years now. I still want one. But, it's just a bit out of reach for me financially. It's worth every dime, but if I don't have it, I don't have it (yet).

However, the SV110ED is also an excellent scope, and at an amazing price. I've also been familiar with it for several years now. Every one I've looked through produces an excellent image for a two-element refractor. Most of the StellarVue scopes are triplets. That is, they have three elements in their objective lens (that is, the "field lens", the bigger one at the end of the telescope that faces the sky.) Coupled with the great design that Vic Maris, owner of StellarVue, gives each of his telescopes, those three element designs produce an image that's as close to optically perfect as it's possible to get. Far better, in my opinion, than designs from his competition (who may turn out occasional scopes that are especially good by chance, but none, to my knowledge, have scopes like StellarVue's where every scope comes out at the apex of optical quality.)

And if my word's not good enough for you, check out what Dennis DiCicco has to say about StellarVue's SV102 telescope in the September, 2013 issue of Sky and Telescope Magazine.

The 110ED is only a "two element" design (two parts to the field lens), but the design is such that it still produces amazing images (like all of the StellarVue scopes.) It is certainly better than my degraded Dobsonian's 8" (202mm) mirror in its current condition, and in the Dobsonian telescope as it is currently constructed.

Better at Half the Size?

There are many reasons a 110mm telescope can outperform a 202mm telescope, even aside from the degraded optics in my Dobson. One is that the Dobson, like all Newtonians, has a secondary mirror in the middle of the light path in the center of the scope. Newtonians have a "spider", which is mechanical elements to hold the secondary mirror in place, which cause diffraction in the image (at least.) The spider legs in my Dobson are relatively thick, being made from cedar shims, in line with John Dobson's original design for this type of scope, which used sections of cedar shingles. I did it not because of optical performance, but to see how they work in a practical sense (which is well--repairs are easy, but optical performance suffers.) I always intended to replace the secondary support with something that caused less effect on the optical image, but time got away from me while I was concentrating on raising my kids, and here we are.

The tube is also quite simple. An optically optimized Newtonian has light baffles inside. Basically these are partitions in the telescope tube that cast shadows everywhere that incoming light is not supposed to be. They are precisely sized to the "cone" of light that the telescope is designed to view, according to the size and placement of optics inside the tube. Instead, I roughed up the the inside surface of my telescope tube, painted it flat black, and called it a day. The tube itself should also be a bit longer to do a better job of keeping stray light from coming in from the side of the front to the eyepiece and focuser.

You get the idea. This is not in any way an optically optimized telescope. It was built to be "good enough". Originally, its performance was excellent when I was at a nice, dark location where the lack of extra tube and baffles wasn't a significant problem. But I did have some nights of viewing ruined by ambient light when I took the telescope into the city for "Sidewalk Astronomy" star parties. Only the brightest objects were visible, and sometimes only when I shaded one side of the end of the telescope from stray light nearby. While I always intended to improve the optical design of my scope, I never actually got around to it.

StellarVue, Right from the Start

The StellarVue won't have that problem. I can count on it being optically optimized right from the get-go. I also know that Vic stands behind his products. If something does come out wrong, or go wrong, he will fix it (needless to say, if it's a result of me doing something terribly wrong, I expect to pay him to fix it. No cleaning optics with steel wool, for example!)

StellarVue has brought more and more of its production of parts in-house over the past several years, too, allowing them greater control over the consistency of the parts they get for producing their products. They can do all the things I never found time to do (baffling, tuning the design), and I get the advantage of it as soon as I put the scope on a mount out under the sky.

Holding and Pointing the SV110ED

As I have said in practically every article on this blog, the best telescope in the world won't do you any good without a mount that's at least equal in quality to the optics of the scope. In fact, I've even advised worrying more about the mount than the optics of your scope. A so-so telescope on a good mounts is far better than an excellent scope on a so-so mount, especially when you're new to astronomy.

I considered getting one of StellarVue's alt-az mounts. These are completely manual mounts that move two directions--up and down (alt, or altitude) and side to side (az, or azimuth movement.) They sell one that would work perfectly, and it's inexpensive for the level of quality. A Dobson mount is a type of alt-az mount.

However, this type of mount has a disadvantage for an observer like myself. I'm going to want to use my 110ED at high levels of magnification. The excellent design of the scope allows it to be used at levels of magnification as high or better than the "rule of thumb" of 100 powers per inch of aperture (depending on observing conditions and the object you're looking at, but I don't want to get into all that now.) Unless you're an experienced observer, this shouldn't be a significant factor for you. The field of view of the 110ED is wide enough that it can be used without any sort of tracking of the sky by your telescope mount up to about 150 powers of magnification. Tracking by hand, with a good eyepiece, will work well.

I'm planning to go higher than that. This is the stuff that my other telescope, the Newtonian on the German Equatorial Mount, would get hauled out for. It has a clock drive, and its movement is oriented to follow the movement of the stars in the sky (really, it's moving opposite the rotation of the Earth.) I decided to get an equatorial mount for my 110ED, too. (I thought about repurposing the mount for my Newtonian, but I actually already have plans for it for a Gregorian telescope I'm building.)

The mount I've bought is the Celestron AVX mount. Stellarvue sells the Celestron line in addition to their own, and they've got many customers before me who've married this mount to this scope, and other comparable StellarVue scopes. It's a German Equatorial, with a clock drive and a guiding computer (normally not an advantage in my book, but I'm willing to tolerate a computer on my telescope if it doesn't get in the way too much. I guess I'm mellowing with age.) Plus...

Plus this mount will be big enough to handle that SV115T once I scrape together the money for it.

Until then, I'm going to have a blast with my new SV110ED. StellarVue says that it will be ready for delivery soon. I can hardly wait. My own StellarVue! Finally!

Expect to see another set of "What Can I See..." articles for telescopes in this size range to complement my series of articles for 60mm telescopes. I will be doing a side-by-side shootout between my StellarVue 110mm scope, a 90mm Meade ETX Maksutov-Cassegrain, and occasionally a 114mm Department Store Newtonian. I may even slip in comparisons to six inch and eight inch telescopes in there.
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